Does using Wi-Fi on iPhone use data?

So after 7 years, I finally took the iPhone plunge. My dad's retirement plan got us discounted, unsold phones last week and we ordered iPhone 4s for the family. I made the rookie mistake of inadvertently leaving the 3G on whenever I wasn't connected to wi-fi, downloading a ton of apps that require internet connection, etc. We got a text saying an abnormal amount of data was used within a week and that ended up being via those mistakes. I've already shut off the cellular data on the phone for the time being but as a newbie to the whole iPhone thing - does leaving the wi-fi on but not 3G/cellular data add to the data plan or does it just work like a laptop and you can use wi-fi wherever?

No it doesn't.